1. Technological Field
The exemplary embodiments described herein relate to an image forming apparatus that has a fixing unit, such as a copier, a printer, a facsimile machine, a multi-functional digital machine, etc.
2. Description of the Related Art
In electrophotography, an image forming unit makes an electrostatic latent image on a photoconductor, develops it using toner, and transfers the developed toner image to a sheet of paper. A fixing unit then fixes the toner image on the paper. The fixing unit has a heated fixing member (a fixing roller, a fixing belt, etc.) and a pressure member (a pressure roller, etc.). When the toner image on the paper passes through a fixing nip formed between the fixing member and the pressure member, the toner image is heated, pressed to the paper, and fixed on the paper.
Some fixing units have a separating plate or a separating claw to separate the paper having a fixed toner image thereon from the fixing member. However separation by the separating plate or the separating claw sometimes damages the surface of the image or the fixing member. Therefore, some fixing units don't have the separating plate or the separating claw. But such fixing units have the following problem.
After the tip of paper goes through the fixing nip, and before it reaches the output paper roller, it is supported and carried by only a fixing nip. Therefore, movement of the tip of the paper is in an unstable state. Especially, because thin paper is soft, thin paper is easily adhered to the fixing member and a paper jam occurs often. Toner is melted by heat of the fixing member and works like glue. The thin paper is adhered to the fixing member or the pressure member by toner, wrapped around the fixing member or the pressure member, and a paper jam occurs. This type of paper jam is called “a wrapping jam”.
As shown in Japanese Patent Publication No 2005-173486 (JP 2005-173486-A), the wrapping jam can be prevented by setting a white space (that means no toner) at the tip of the paper, but too large of a white space means a lack of image information. Therefore the image forming apparatus shown in Japanese Patent Publication No 2005-173486 (JP 2005-173486-A) has a separating claw that can be moved to contact and depart from the fixing member and the pressure member. Further, the size of the white space in the direction of paper moving can be set. When the size of the white space is more than a predetermined value, the separating claw departs from the fixing member and the pressure member. When the size of the white space is less than a predetermined value, the separating claw contacts the fixing member and the pressure member, thus preventing a wrapping jam. However, when the separating claw is used, the problems related to the separating claw occur. Moreover, this technology does not consider time degradation. As time goes on, the wrapping jam occurs more frequently. So, even in this technology, there is probability that the wrapping jam occurs.